My present invention relates generally to radioisotope microspheres. More particularly, the invention relates to a novel process and associated apparatus for rapidly and efficiently producing uniformly sized microspheres.
Radioisotope microspheres are normally formed by the sol-gel process which is a wet-chemistry technique. A starting (nitrate) compound is suitably dissolved, the metal hydroxide precipitate is washed, and crystallite (oxide) growth is controlled and concentrated by heat and evaporation. The resulting sol-gel of a suitable consistency or viscosity is formed into microspheres which are then calcined in a furnace at approximately 1500.degree. C to achieve near-theoretical density. The sol-gel process has proven advantages including high density microsphere formation capability, reasonable economy, wide reproducible size range, and minimal airborne scattering or loss of radioactive particles.
In the conventional process of forming sol-gel microspheres, the sol-gel is fed at a flow rate according to its consistency or viscosity along with a flow of alcohol. The sol-gel is contained in an ordinary syringe mounted on an infusion pump which can be varied to drive the syringe plunger at a desired rate. The sol-gel output from the syringe is fed through a flexible tubing into a squared tip hypodermic needle, the end of the tubing being connected to the upper end of the needle by a suitably tapered connector. The tapered connector is installed in a coupling collar at the top of a relatively long microsphere formation nozzle so that the hypodermic needle is concentrically mounted and extends down within the tubular nozzle. The length of the needle is, for example, about 2 inches long and the nozzle is about 6 inches long, with the lower 1 inch of the nozzle immersed in a microsphere drying alcohol column.
The coupling collar has a laterally connecting passageway opening adjacent to the tapered wall of the tapered connector. Alcohol is fed in the passageway and flows around the tapered wall of the tapered connector and down the outside of its connected needle. The alcohol flows pass the end of the hypodermic needle and generally keeps the surrounding tubular nozzle filled with its flow. Microsphere formation is accomplished by the alcohol flow acting on the sol-gel emitted at the end of the needle. The alcohol flow inevitably fluctuates, however, and produces a considerable effect on the formation of the microspheres. Size variation is a major result and this causes a high rejection rate of unsuitably sized microspheres. Provision of a proper alcohol feed with the sol-gel feed precludes the satisfactory use of more than one needle within a microsphere formation nozzle. Thus, production of microspheres in quantity is infeasible with this equipment. The inherent static operating conditions of the equipment also account for a certain amount of the variations in sol-gel viscosity and needle plugging by the sol-gel.